This application is based on and claims priority from Japanese Patent Application Nos. H11-264172 filed on Sep. 17, 1999 and H11-270072 filed on Sep. 24, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a bright surface structure formed on a member made from metal or resin and having a good ductility and corrosion resistance, and a method for manufacturing thereof.
2. Description of Related Art
To brighten a surface of a metal or a resin member, conventionally an electro plating, such as an electro chromium plating is used. By this method, a high reflectivity, a gloss of a high degree having an appearance of deepness, a good wearing resistance and a good corrosion resistance are obtained. This surface brightening treatment is applied to a metal member such as an aluminum wheel of an automobile, and a resin member such as a front grille, a garnish and an emblem of an automobile for lightening the member.
Since the electro plating needs an environmental countermeasure, recently various methods replacing the electro plating have been investigated. As one substituting method, with the aluminum wheel of an automobile, a surface of the wheel is ground to a smooth surface. However, the grinding is not only accompanied by an increase in manufacturing cost but also inapplicable to an inside surface of ventilation holes of the wheel.
Japanese Patent Publications H04-131232 and H10-130822 propose the following improving methods replacing the grinding method:
(1) Japanese Patent Publication H04-131232
After a metal member is surface-treated, the member is coated with an under coat, then a thin chromium film formed by sputtering, and then a top coat. A surface of the thin chromium film operates as a reflective surface. In the surface treatment, the surface of the member is shot-blasted and coated with a clear resin by powder coating. As the under coat, a clear resin film is formed. The chromium film is formed using chromium. The top coat is a clear coat that protects bright surface of the chromium film.
However, the method described in Japanese Patent Publication H04-131232 presents a problem of the thin chromium film cracking, though thin chromium film presents an external appearance similar to electro chromium plating. Because the residual stress of the chromium film is large, cracking is caused during sputtering and after the top coat is coated, or after the top coat is coated on the chromium film and dried. When the thickness of the chromium film is equal to or greater than 0.04 xcexcm, the crack initiation is severe. The residual stress is so significant that when a stress generated in the top coat during abrasion test and weather resistance test is transmitted to the chromium film, cracking easily occurs in the chromium film. Further, when a solvent penetrates the chromium film during coating and/or drying the top coat, cracking easily occurs in the chromium film.
If the chromium film is reduced in thickness in order to suppress generation of the cracks in the chromium film, the external appearance similar to electro chromium plating cannot be obtained any longer because an under wheel surface is seen through the thin chromium coat and the brightness of chromium film is reduced.
(2) Japanese Patent Publication H10-130822
After a powder coating is applied to an aluminum forging part, an under coat, then a thin aluminum film formed by sputtering, and then a top coat are formed. A surface of the thin aluminum film operates as a reflective surface.
However, the method described in Japanese Patent Publication H10-130822 presents a problem that corrosion resistance is relatively low, though no cracks occur even if the aluminum film has a thickness of about 0.08 xcexcm, since aluminum film has a high ductility. Though the top resin coat formed on the aluminum film protects the aluminum film, once the top coat is damaged during lifetime of an automobile, corrosion propagates from the damaged top coat. When the aluminum film is removed due to the corrosion, the brightness is gone.
The present invention provides for a bright surface structure formed on a metal or resin member having an external surface similar to an electro chromium plating, and a good ductility and corrosion resistance, and a method for manufacturing thereof.